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Soil sampling and interpretation
Dan M. Sullivan Oregon State University
Crop & Soil Science
Soil School Multnomah Conservation Districts
April 4, 2015
Why do chemical analysis of soils? • Establish baseline nutrient status
• Determine application rates
• Assess pH and need for liming
• Measure changes over time
• Avoid excess nutrient application
How often to test soil
• Preplant for perennial crops, very important. • Some nutrients (P and K) and lime application
work best if incorporated into top 6 inches of soil (preplant)
• Usually, soil testing every 4 years or so is sufficient
Soil probe
• Collect same amount of soil from each depth
• Known sampling depth • Easy to clean out
between samples
Photo: OSU Extension
Use a garden shovel? (No!!)
• Depth unknown • More soil from top than
from bottom of hole • May be zinc plated
Photo: Virginia Tech
EC 628
• Goal is to obtain uniform
sample to prescribed depth • Nutrients usually higher
near soil surface • When soil is rocky, a
“sharpshooter” (at left) may be your best tool
• http://www.naptprogram.org/pap • Voluntary soil testing quality control program supervised by
Soil Science Society of America • About 10 labs in West were “certified” in 2014 • Based on annual performance in accurate analysis of
“double-blind” soil samples • Must use NAPT-PAP lab when sampling under cost-share
agreement with NRCS for nutrient management
Suggestions for best value from soil testing
• Review OSU fertilizer/nutrient guide before you sample. Note sampling depth(s) and recommended soil tests
• Keep soil sampling method consistent (depth, month)
• Send samples to labs that are certified by North American Proficiency Testing Program http://www.naptprogram.org/pap/labs
• Keep records field by field in a database
Commercial soil test report
This lab categorizes soil test values in five categories: very low, low, medium, high and very high. But who decided what was “low” and “high”? How much fertilizer or lime do I apply?
• OSU EC 1478-E • Most recent summary of applicable soil test methods
used in the Pacific Northwest • General reference: “approved” method, general
interpretation: low, medium, high, excess • More specific information provided in crop/region
specific nutrient management guides
Secrets of soil test calibration
• Soil test value related to crop yield response for soils in a specific region
• Soil test values by themselves (no calibration to crop yield response) are interesting, but not very useful
Soil Testing Example (P)
• Standard” agricultural soil tests extract a portion of total soil phosphorus
• An effective soil test extractant mimics a plant root (extracts some of the “available” P)
Available Unavailable Soluble
Soil test value
• “Index” value • Just like you can’t compare Dow vs. NASDAQ
stock indexes, you can’t compare soil test P values obtained with different soil test extractants.
• To have value in diagnosing nutrient need, soil test value needs to be calibrated to crop response to nutrient addition
Soil test calibration experiments performed by OSU and land grant
universities • Conducted experiments at field sites with low
to high soil test values • Many farmers, many soils, one crop at a time • Determined what soil test value is adequate or
sufficient to support maximum crop production
Soil test calibration field experiments
• Each experiment at each field site had two fertilizer treatments (replicated within field): – No nutrient added – High rate of nutrient added
• Response to nutrient expressed as relative yield (%): (Crop yield without fertilizer)/(crop yield with fertilizer) x 100
• All other crop management factors optimized, uniform
Soil test interpretations are based on calibration curves developed from field experiments
Source: OSU EC 1478-E (2011)
Crop specific P recommendation (found in fertilizer or nutrient management guide for
specific crop in a region)
In: OSU Fertilizer Guide for Pastures (western OR). Revised 2000.
Soil analyses: west of Cascades (pH < 7; precipitation 30 to 50+ inches)
• soil pH • lime requirement: SMP buffer test • Bray P1 phosphorus • Exchangeable cations (Ca, Mg, K ) • hot-water extractable B • Post-harvest NO3-N?
– Sept 1-Oct 15
Interesting but probably not essential
• Ammonium-N (NH4-N) • Sulfate-S (SO4-S) • Percent base saturation • Cation exchange capacity (CEC) • Nutrient ratios • Soil texture • Mineralizable N, total N • Soil health score (Haney test) • Organic matter or soil carbon
– Sometimes useful for long term monitoring
General management for western Oregon acid soils
• Based on soil testing: – Apply lime to raise soil pH to 6.5 for all crops (except
blueberries, red maple trees, and other acid loving plants) – Use fertilizer or compost to supply P, K, S, and
micronutrients
• Apply N fertilizer based on crop demand (from university guides). Soil tests not used to predict crop N need.
• Reduce N fertilizer rate when following legume cover crop
• Reduce N fertilizer when organic fertilizer/compost applied often in previous 3 to 5 yr.
Nitrogen
• N fertilizer application is usually essential • Soil testing is not used to predict crop N need • Nitrogen recommendations in OSU guides are
based on crop response to N fertilizer in field trials
Organic Nitrogen
Leaching loss to groundwater
Nitrate NO3
Ammonium-N NH4
Crop N Uptake
Ammonia loss
NH3 gas Denitrification loss
N2 or N2O gas
Soil Sullivan. EM 8954-E. OSU Extension.
Soil organic matter
• More N release from soils with frequent manure or compost application
• OSU nutrient management guides already allow for typical N release from soil organic matter
• So, soil organic matter number on your soil test report is interesting, but probably not useful to N management decision
Crop specific guides will give guidance on timing of N fertilizer application
(example: pastures)
In: OSU FG-63. Fertilizer Guide for Pastures (western OR). Revised 2000.
Sulfur • If soil S is low, other nutrients are not used efficiently • But, like N, soil testing won’t tell you how much S to
add • Typical recommendation: 0.5 to 1 lb sulfate sulfur
(S) per per thousand sq ft. each year • S fertilizers include ammonium sulfate, gypsum, or
sulpomag (sometimes called KMag) • If you apply compost or manure, S fertilizer usually
not needed
Three most important nutrient management decisions based on soil
testing • Apply lime as needed • Apply lime as needed • Apply lime as needed
What is pH?
pH units are logarithmic, like the Richter scale for earthquake severity A pH of 5 is ten times more acidic than a pH of 6
What is the effect of soil pH on plants?
When soil is too acidic (low pH): • Plant root growth and crop yield is reduced • Legumes are especially sensitive to acid soil Crops differ in their tolerance to acid soil (low pH)
Doerge, 1980s, Willamette Valley, Agronomy Journal
Lime to avoid a crop yield “crash landing” Example: alfalfa
n
OSU EM 9057 (2013)
See also OSU Guide EM 9061 (2013) Soil Acidity in Oregon: Understanding and Using Concepts for Crop Production
When is liming needed?
• Liming “fixes” (increases) soil pH slowly • It takes 6 to 12 months for full reaction with
soil • Lime has to be tilled into soil to get best
results • So plan ahead • If you are planting a perennial agronomic
crop, apply enough lime for the life of the crop
How much lime to apply?
• Goal is to keep soil pH from getting too low. • Use soil testing to determine lime need. • Get on a schedule, like going to the dentist… • Consider applying modest rate of lime every 4
years, rather than a high rate of lime every 10 years.
2 to 5 ton of 100-score lime per acre is required to change soil pH by 1 unit
(for example, from pH 5 to pH 6)
One ton of lime per acre =
About
5 lb lime per 100 square ft
Or
50 lb lime per thousand square ft.
Source: OSU EM 9057
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